Displacement reaction for producing NF4 PF6

ABSTRACT

A method of producing NF 4  PF 6  by a displacement reaction between NF 4  BF 4  and PF 5 .

The invention herein described was made in the course of or under acontract or subcontract thereunder, (or grant) with the United StatesNavy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 732,275 filedOct. 14, 1976, and abandoned Dec. 5, 1977, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of producing compositions and isparticularly directed to a method of producing NF₄ PF₆ by a displacementreaction between NF₄ BF₄ and PF₅.

2. Description of the Prior Art

NF₄ ⁺ salts are the key ingredients for solid propellant NF₃ --F₂ gasgenerators, such as that disclosed by D. Pilipovich in U.S. Pat. No.3,963,542, for chemical HF-DF lasers. Whereas NF₄ SbF₆ and NF₄ AsF₆ canbe prepared with relative ease, according to the methods taught by W. E.Tolberg et al, in U.S. Pat. No. 3,708,570, and K. O. Christe et al, inU.S. Pat. No. 3,503,719, these compounds suffer from the disadvantage ofcontaining a relatively heavy anion, thus decreasing their performancein an NF₃ --F₂ gas generator. This disadvantage can be overcome byreplacing the SbF₆ ⁻ or AsF₆ ⁻ anion by the lighter PF₆ ⁻ anion. Theexistence of this salt has previously been claimed by Tolbert et al inU. S. Pat. No. 3,708,570, but their production process was soinefficient that they could not isolate an amount of material sufficientfor its isolation, identification and characterization.

BRIEF SUMMARY AND OBJECTS OF THE INVENTION

This problem of synthesizing NF₄ PF₆ is overcome by the presentinvention. The method of the present invention involves a displacementreaction between the readily available NF₄ BF₄ and PF₅ according to:

    NF.sub.4 BF.sub.4 +PF.sub.5 →NF.sub.4 PF.sub.6 +BF.sub.3

applicants have found that the displacement reaction can be carried outat any temperature above the melting point of PF₅ (-94° C.) and belowthe decomposition temperature of NF₄ PF₆ (above 245° C.). Moreover, thepressure is not essential and is given by the reaction temperature (thatis, the vapor pressure of PF₅).

This method provides NF₄ PF₆ of high purity.

Accordingly, it is an object of the present invention to provide animproved process for the production of NF₄ PF₆.

This and other objects and features of the present invention will beapparent from the following examples.

DETAILED DESCRIPTION OF THE INVENTION

In a typical experiment, pure NF₄ BF₄ (2.07 mmol) was combined at -196°C. with an excess of PF₅ (40.01 mmol) in a 10-ml 316 stainless steelcylinder. The mixture was kept at 25° C. for 64 h. The volatilematerials were removed in vacuo and separated by fractionalcondensation. They consisted of BF₃ (2.05 mmol) and unreacted PF₅ (37.93mmol). The white solid residue had gained 120 mg in weight. Based on theabove material balance, the conversion of NF₄ BF₄ to NF₄ PF₆ wasessentially complete. This was further confirmed by vibrationalspectroscopy which showed the solid to be NF₄ PF₆ containing nodetectable amounts of NF₄ BF₄.

The salt NF₄ PF₆ is a white, crystalline, hydroscopic solid, stable atroom temperature, but rapidly decomposing at 245° C. Its characteristicx-ray diffraction powder pattern is listed in Table I. Its vibrationalspectrum is listed in Table II and establishes the ionic nature of thesalt, i.e. the presence of discrete NF₄ ⁺ cations and PF₆ ⁻ anions. Thiswas further confirmed by ¹⁹ F nmr spectroscopy in HF solution whichshowed the triplet (J_(NF) =230 Hz at φ=-217) characteristic for NF₄ ⁺.

                  TABLE I                                                         ______________________________________                                        X-RAY POWDER DATA FOR NF.sub.4 PF.sub.6.sup.a                                 d obsd      d calcd     Int       h k l                                       ______________________________________                                        5.40        5.36        ms        1 1 0                                       4.55        4.53        s         1 0 1                                       3.91        3.89        vs        1 1 1                                       3.79        3.79        s         2 0 0                                       2.91        2.91        ms        211                                         2.65        2.65        m         1 0 2                                       2.40        2.40        vw        3 1 0                                       2.307       2.305       m         3 0 1                                       2.204       2.205       vw        3 1 1                                       2.171       2.171       mw        2 1 2                                       1.882       1.883       ms        3 0 2,0 0 3                                 1.825       1.827       vw        3 1 2,1 0 3                                 1.784       1.785       w         3 3 0                                       1.747       1.747       mw        4 1 1                                       1.685       1.685       w         3 2 2,2 0 3                                 1.646       1.646       w         2 1 3                                       1.622       1.622       w         4 2 1                                       1.536       1.540       vw        4 1 2                                       1.485       1.486       vw        5 1 0                                       1.464       1.463       vw        5 0 1                                       1.437       1.437       w         5 1 1                                       1.408       1.407       vw        5 2 0                                       1.365       1.365       vw        5 2 1,1 1 4                                 1.333       1.335       w         5 0 2                                       1.318       1.319       vw        4 4 0                                       1.302       1.304       vw        2 1 4                                       1.259       1.259       w         4 2 3                                       1.214       1.216       w         6 1 1                                       ______________________________________                                         .sup.a Tetragonal, a = 7.577, c = 5.653A, Cu K.sub.α  radiation Ni      filter                                                                   

                  TABLE II                                                        ______________________________________                                        VIBRATIONAL SPECTRUM OF NF.sub.4 PF.sub.6                                     Obsd Frequency (cm.sup.-1)                                                    Ir           Raman      Assignments for NF.sub.4.sup.+ (T.sub.d)              ______________________________________                                        2380 vw                                                                                               2ν.sub.3 (A.sub.1 + E + F.sub.2) = 2320            2320 w                                                                        2005 w                  ν.sub.1 + ν.sub.3 (F.sub.2) = 2008              1765 w                  ν.sub.3 + ν.sub.4 (A.sub.1 + E + F.sub.2) =                             1769                                                  1457 w                  ν.sub.1 + ν.sub.4 (F.sub.2) = 1457              1221 mw                 2ν.sub.4 (A.sub.1 + E + F.sub.2) = 1218            1166 vs                                                                                    1168(1.5)  ν.sub.3 (F.sub.2)                                               1150(0.8)                                                        1135 vw                                                                       1056 vw                 ν.sub.2 + ν.sub.4 (F.sub.1 + F.sub.2) =                                 1049                                                                880(0.2)  2ν.sub.2 (A.sub.1 +  A.sub.2 + E) = 880                          849(8.2)  ν.sub.1 (A.sub.1)                                   611 m                                                                                      609(7.4)  ν.sub.4 (F.sub.2)                                   608 m                                                                                      441(2.9)  ν.sub.2 (E)                                        ______________________________________                                                                Assignments for PF.sub.6.sup.-  (O.sub.h)             ______________________________________                                        1590 w                  ν.sub.1 + ν.sub.3 (F.sub.1u) = 1590             1414 w                  ν.sub.2 + ν.sub.3 (F.sub.1u + F.sub.2u) =                               1413                                                  1308 vw                 ν.sub.1 + ν.sub.4 (F.sub.1u) = 1307              842 vs                                                                                     838(1.5)  ν.sub.3 (F.sub.1u)                                  789 w                                                                         749 w        748(10)   ν.sub.1 (A.sub.1g)                                               571(0.8)  ν.sub.2 (E.sub.g)                                   559 s                  ν.sub.4 (F.sub.1u)                                  474 vw                                                                                     469(1.2)  ν.sub.5 (F.sub.2g)                                 ______________________________________                                    

Obviously, numerous variations and modifications may be made withoutdeparting from the present invention. Accordingly, it should be clearlyunderstood that the form of the present invention described above isillustrative only and is not intended to limit the scope of the presentinvention.

We claim:
 1. A process for the production of NF₄ PF₆ characterized bycombining NF₄ BF₄ with an excess of PF₅ at above about -196° C.,reacting the reactants while warming the reaction system to atemperature less than about 25° C. and removing the volatile reactionproducts by pumping.